The present invention relates to a heat-sensitive transfer medium for use in thermal transfer apparatuses such as thermal printer and facsimile. More particularly, it relates to a heat-sensitive transfer medium capable of providing print images, such as letters, symbols and patterns, which have a metallic luster such as gold or silver.
A heat-sensitive transfer medium which has been widely used heretofore is that wherein a heat-meltable ink layer which is melted in a prescribed temperature is provided on a support, for instance, having a thickness of 3 to 12 .mu.m.
The mechanism of printing using such heat-sensitive transfer medium is as follows: A thermal head is brought into contact with the back surface of the support of the transfer medium. When plural heating elements of the thermal head are selectively activated on the basis of signals for printing to generate heat, portions of the heat-meltable ink layer which are positioned on the heated portions of the support are melted and transferred to a receiving medium, such as plain paper, which is brought into contact with the heat-meltable ink layer, providing transfer images of the heat-meltable ink corresponding to the printing signals on the receiving medium. Thus, the use of the heat-sensitive transfer medium makes possible printing onto a plain paper.
The heat-meltable ink layer used in the conventional heat-sensitive transfer medium is usually an ink layer wherein pigments such as carbon black are mixed with a heat-meltable vehicle such as wax. For this reason, the color of the transfer image formed on the receiving medium is restricted to that of the pigment used in the heat-meltable ink layer. Although a heat-meltable ink layer wherein a metal powder is used as a pigment is known, it cannot absolutely provide a transfer image having an excellent metallic luster such as specular gloss.
On the other hand, a plastic film is usually used as a support for the above-mentioned heat-sensitive transfer medium. However, usual plastic films have a melting or softening temperature of 200.degree. to 300.degree. C. at the highest and also a heat deformation temperature of 100.degree. C. at the highest, while the surface temperature of the thermal head goes up to high temperatures of 300.degree. to 400.degree. C. When such plastic film as the support is heated with the thermal head during printing, the so-called "hot-sticking phenomenon" occurs. The hot-sticking phenomenon involves disadvantages such as sticking of the thermal head to the plastic film (hereinafter referred simply to as "sticking"), which causes hindering in the feeding of the transfer medium; and attaching of some melts (hereinafter referred to as "sticking-dust") of the plastic film to the thermal head.
In order to prevent such hot-sticking phenomenon, heretofore, an attempt that a sticking-preventive layer was provided on the back surface of the plastic film which is to be brought into contact with the thermal head was made. As the sticking-preventive layer, there were proposed a metal layer, a heat-resistant resin layer, a layer composed of benztriazole, an ethyl cellulose layer containing sodium stearyl sulfate and a polyester resin layer containing stearic acid. However, these sticking-preventive layers had drawbacks that when the thickness was small, a sufficient sticking-preventive effect was not attained, and when the thickness was large, the heat-sensitivity was reduced due to an increase in heat capacity and the sticking-preventive layer itself rather causes sticking and sticking-dust.
It is an object of the present invention to provide a heat-sensitive transfer medium capable of producing transfer images with a variety of metallic lusters including a specular gloss and an appropriately matted metallic luster being remarkably artistic.
Another object of the present invention is to provide a heat-sensitive transfer medium improved in sticking-preventive property as well as the above-mentioned ability of producing transfer images with an excellent metallic luster.
These and other objects of the invetnion will become apparent from the description hereinafter.